1. Field of the Invention
The present invention relates to an electro-acoustic transducer and, in particular, relates to an electro-acoustic transducer which radiates a sound wave into a medium such as water.
2. Description of the Related Art
An electro-acoustic transducer which radiates a sound wave into a medium such as water is installed, for example, in a transmitter of sonar used for a marine resource search, an ocean current investigation or the like. Since a sound wave in a low frequency band can be propagated long-range in the water, the electro-acoustic transducer capable to radiate the sound wave in the low frequency band is requested. Moreover, since the electro-acoustic transducer is usually installed in a ship or an airplane, a small-sized electro-acoustic transducer with the high power efficiency is requested.
In order to cope with the above mentioned requests, the electro-acoustic transducers of various structures have been proposed. For example, Japanese Patent Application Publication No. 62-176399 discloses a bolted Langevin type electro-acoustic transducer in which a pillar-shaped vibrator including laminated piezoelectric ceramic plate is interposed between a front mass and a rear mass, and the front mass and the rear mass are fastened with a bolt. The electro-acoustic transducer radiates a sound wave in a medium in a longitudinal vibration mode. Since an electro-mechanical coupling coefficient of the longitudinal vibration mode is relatively large, the electro-acoustic transducer can radiate a strong sound wave from the front mass.
JP05-219588 A discloses an electro-acoustic transducer having an acoustic radiation plate in which a vibrator including a piezoelectric ceramics or the like is arranged. The electro-acoustic transducer radiates a sound wave in a bending vibration mode in a medium. Since a resonance frequency of the bending vibration mode is lower than a resonance frequency of the longitudinal vibration mode, this type of the electro-acoustic transducer can lower a frequency of the output sound wave. Moreover, a ratio of a sound radiation area to a total apparatus surface area in the electro-acoustic transducer is higher than that of the electro-acoustic transducer disclosed in JP62-176399 A. Accordingly, the electro-acoustic transducer disclosed in JP05-219588 A can be smaller and lighter than the electro-acoustic transducer disclosed in JP62-176399 A.
In general, the lowest frequency which can be output by an electro-acoustic transducer depends on the lowest resonance frequency of a vibration plate. A resonance frequency of a longitudinal vibration mode depends on weights of a front mass and a rear mass, and depends on stiffness of a pillar-shaped vibrator. Accordingly, in order to lower an output frequency of the electro-acoustic transducer in the longitudinal vibration mode, it is necessary to weight the front mass and the rear mass or to lengthen the pillar-shaped vibrator. That is, the electro-acoustic transducer disclosed in JP62-176399 A cannot cope with both lowering the output sound wave frequency and reducing a size and weight thereof.
The electro-acoustic transducer disclosed in JP05-219588 A adopts a structure in which the vibrator is directly installed in the acoustic radiation plate, and the acoustic radiation plates are fixed at the edge portions thereof. In the acoustic radiation plate, the edge portion acts as a node of the vibration. Vibration amplitude of the acoustic radiation plate may be large at a central portion, but is quite small or almost zero at the vicinity of the fixed portion. Since the excluded medium volume is corresponding to the vibration amplitude of the acoustic radiation plate, the electro-acoustic transducer disclosed in JP05-219588 A has low electro-acoustic transduction efficiency. Since the heavy vibrator is directly installed in the acoustic radiation plate in case of the electro-acoustic transducer disclosed in JP05-219588 A, weight of the acoustic radiation plate increases. Due to the heavy acoustic radiation plate, a resonance frequency bandwidth of the acoustic radiation plate in the bending vibration mode becomes very narrow. Accordingly, the electro-acoustic transducer disclosed in JP05-219588 A can not radiate a broadband sound wave.